This invention relates to an improvement in bearings and bearing rolling elements, and particularly in roller bearings and rollers for such bearings. More particularly, the invention relates to an improved recirculating roller bearing including an improved roller.
Recirculating roller bearings are well known in the art, and are described in U.S. Pat. Nos. 3,003,828 and 3,341,262. These bearings are used to provide substantially friction-free movement between two surfaces, at least one of which is linearly movable with respect to the other. A recirculating roller bearing is generally an elongated device, with two opposed flat faces, which can be secured at one of its faces to one surface, while its opposite face rests on an opposed movable surface.
A recirculating roller bearing generally includes a continuous raceway along its race body which is formed by a lower race, an upper race, and two opposed end return chambers that interconnect the upper and lower races. In a preferred form, the upper race is formed as a channel between two shoulders. The channel receives the rollers to avoid their contacting the adjacent upper surface. The lower race is at least partially open to allow rolling contact between the rollers and the adjacent lower surface. In cross-section, at a point between the end return chambers, the race body has a U-shaped configuration, the legs of the U being the shoulders that form the upper-race channel. The upper-race shoulders bear the load at the bearing's upper face, and the rollers disposed in the lower race bear the load at the bearing's lower face.
As used herein, the "upper portion" or the "top" of the bearing indicates that portion of the bearing which is immovably affixed to a surface. That upper portion is formed by two outer shoulders which over-extend the rollers and, hence, do not allow the rollers in the upper return chamber to contact the adjacent surface. Likewise, the "lower portion" or "bottom" of the bearing refers to that surface of the bearing which has exposed rollers surfaces and which movably contacts a surface which comes in contact with the bearing. It should be understood, however, that the recirculating bearing of the present invention can be operated in any orientation, and terms referring to the "top" or "bottom" are used for convenience only.
In a conventional recirculating roller bearing, the rollers normally have a dumbbell configuration with two cylindrical end members being joined by a mediate portion of reduced diameter. Those rollers are often referred to as center-guided rollers. For optimum performance, it is desirable that the rollers circulate in the race along a path precisely perpendicular to the axis of the race. This "straight rolling," is the optimum rolling condition because friction and its adverse effects, such as heat, wear, and the like, are kept to a minimum. Often, however, the rollers do not travel precisely perpendicular to the axis of the race. This condition, called "roller skewing," raises the level of friction in the system in two ways: first, an inner edge of the cylindrical end member registers with the guide in the lower race hence a mechanical friction developes as the two surfaces rub together; and second, a corrective force in the form of rotational torque is required to realign the roller. The total force needed to overcome the increased friction caused by roller skewing is called correctional torque, which is the total amount of torque needed to realign the skewed roller.
Roller skewing is a random phenomenon which is present in recirculating bearings regardless of load. However the incidence of this event increases with load so that, while a roller under a light load will skew, a roller under a heavier load will skew more often.
Generally, the present invention provides an improved recirculating roller bearing and roller therefor which more effectively neutralizes the effects of roller skewing by reducing the amount of corrective force needed to realign a roller into a straight rolling trajectory. In accordance with the invention, an improved recirculating roller bearing is provided having a race body with a continuous raceway in which a plurality of rollers circulate. The race body has upper and lower race channels and end return chambers at either end. The rollers are retained in the raceway in any suitable fashion. The rollers have two cylindrical end members joined by a mediate portion of reduced diameter. Each cylindrical end member has an axial bore extending from each end toward the mediate portion.
The inclusion of an axial bore in an otherwise solid cylindrical end member causes a shift in the distribution of mass in the roller toward the center. The reduced mass in the extremities results in a corresponding reduction in the amount of correctional torque needed to realign a skewed roller.
The axial bore described above yields two further advantages. First, under extreme loads, it has been found that the load tends to be concentrated at the outer ends of the rollers. This load concentration results from the distortion of the shape of the lower race under weight. The axial bore permits the rollers to distort more easily at the point of heaviest loading thus distributing the load more evenly.
Second, as a result of minor, but unavoidable, variations in the surface to which the recirculating bearing is attached, a greater percentage of the total load is often borne by one of the cylindrical end members. The axial bore permits the cylindrical end member bearing the greater load to distribute that load away from the roller extremities thus reducing wear and permanent deformation.